Blender 2.8: Why cannot I colour on the same plane?
I've had the same issue. 
There is the ability to select your drawing pane, and lock strokes/fills to that pane. Depending on which build of 2.8 you have, this has moved, so check the image below. 

For my selection, to lock it to the same pane as the Ink Strokes, I needed to select Side (Y-Z). In your screencap I can see you have it set to view, so that if you rotate around your scene you'll move the drawing pane. 
Selecting one from the drawing pane dropdown will lock it down for you.